Methods of producing cement clinker in which a thermal secondary treatment of largely calcined raw material takes place before the actual combustion stage are known in various forms. In one of these known methods (DE-OS-32 37 343) a special heating unit for the calcined material is arranged between the calcining zone and the final combustion zone. In this heating unit the quantity of heat required for rapid heating of the material should be released in measured amounts by setting an adapted heat performance by means of a plurality of fuel supplies distributed over the heating unit. As a result the calcined material should be heated to the temperature at which elite formation begins, which can be on average about 1250.degree. C., but without a temperature occurring in only a part of the stream of material for burning which would facilitate the formation of quantities of melt. One possible type of construction of the heating unit which is indicated is the use of a fluidised bed reactor with overflowing fluidised bed; however, a heating unit is preferred which is constructed as a substantially vertical shaft-like suspension reactor. If with this known method the heating unit is provided--as mentioned--with an extremely large quantity of fuel and/or air supply points, then this results in extremely complicated adjustment for the desired homogeneous temperature profile if undesirable melt formation is to be avoided.
The object of the invention is to make further improvements to conventional cement clinker process with a relatively simple construction and operation wherein rapid heating of previously calcined material and spontaneous heat compensation between the material and the treatment gas in the first stage of the clinker burning zone is ensured as well as a particularly easily controllable clinker finishing burn of the heated material in the second stage.